2 Messrs. Wright and Thompson on the Determination of 
decrement* due to a given increase in strength of the other 
fluid, then E and e, the electromotive forces of the cell when 
containing the two stronger and the two weaker fluids respec- 
tively, are related so that, within experimental-error limits, 
H=e+a—b. 
Tt follows from this fact, and from the circumstance that in 
many cases the values of a and 6 are considerable fractions of 
e, that the great majority of the valuations hitherto made and 
published of the electromotive forces of various two-fluid cells 
cannot be properly compared with the heat-evolutions due to 
the net chemical actions taking place in the cells ; because no. 
accurate valuations having been made of the strengths of the 
solutions used, and the precise character of the plate-surfaces 
employed not having been noted, the values of the terms a 
and } cannot be calculated with precision so as to enable the 
values of H to be deduced for some given standard-strength 
of solution (e. g. for strength MSO, 100H,0, or MCI, 100H,O, 
&c.). Differences of several centivolts, and even decivolts, 
exist between the values assigned to certain voltaic combi- 
nations by different observers, mainly due to this cause. 
Accordingly it becomes necessary to repeat most of the work 
of previous experimenters with due regard to these points 
before comparisons can be satisfactorily instituted. A large 
number of cells have therefore been examined during the last 
four years, under strictly comparable conditions, in order to 
obtain sufficiently accurate values for the purpose of tracing 
out the correlations subsisting in different cases between the 
heat-evolution due to the net chemical change and the maxi- 
mum H.M.F’. actually developed, when so small a current 
flows as to cause no appreciable diminution by “ polarization.” 
All these observitions completely confirm the conclusions 
already arrived at, that Volta’s law of summation holds within 
the limits of experimental error, and that the effect of a given 
alteration in the molecular strength of a saline solution sur- 
rounding one of the plates of a two-fluid cell is sensibly indepen- 
dent of the nature of the other fluid and the other plate. It 
hence becomes possible to assign numerical values or constants 
to given metals immersed in’ solutions of their salts of given 
nature and strength, such that the E.M.F. of a voltaic com- 
bination formed by combining two such immersed metals (by 
allowing the two solutions to interdiffuse, e. g. ina Raoult cell 
* With cells set up with acid fluids (e. g. dilute sulphuric acid) sur- 
rounding the plate acquiring the lower potential, increasing the strength 
of the acid fluid not infrequently causes an increment, and not a decre- 
ment, in the E.M.F. of the combination; e.g. in Daniell cells (§§ 112-114, 
Part) Va): 
